617. Merge Two Binary Trees

余分なスペースを使用したバージョンは比較的明確です.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
        return merge(t1,t2);
    }
    private  TreeNode merge (TreeNode t1, TreeNode t2)
    {
        if(t1==null&&t2==null) return null;
        TreeNode newNode = new TreeNode(0);
        if(t1==null)
            newNode.val=t2.val;
        else if(t2==null)
            newNode.val=t1.val;
        else
        newNode.val=t1.val+t2.val;
        newNode.left=merge(t1==null?t1:t1.left,t2==null?t2:t2.left);
        newNode.right=merge(t1==null?t1:t1.right,t2==null?t2:t2.right);
        return newNode;
            
        
    }
}

余分なスペースを使用しないバージョンで、直接つなぎ合わせます.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
        return merge(t1,t2);
    }
    private  TreeNode merge (TreeNode t1, TreeNode t2)
    {
        if(t1==null&&t2==null) return null;
        if(t1==null)
            return t2;
        else if(t2==null)
            return t1;
        else
        t1.val=t1.val+t2.val;
        t1.left=merge(t1.left,t2.left);
        t1.right=merge(t1.right,t2.right);
        return t1;
            
        
    }
}